ixgbe: limit setting speed to only one at a time for QSFP modules

QSFP+ modules do not support auto negotiation and should advertise only
one speed at a time.

This patch adds logic in ethtool to allow setting and reporting the
advertised speed at either 1Gbps or 10Gbps, but not both. Also limits
the speed set in ixgbe_sfp_link_config_subtask() to highest supported.
Previously the link was set to whatever the supported speeds were.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>